A prior art spiral oven 2 used for cooking food products is schematically illustrated in FIG. 1. The prior art spiral oven 2 comprises: a housing 4 having a lower cooking chamber 6 and an upper circulation and heating chamber 8; a spiral conveyor belt 10 within the cooking chamber 6 for carrying the food product during the cooking process; an upper cooking chamber ceiling 12 which separates the cooking chamber 6 from the upper circulation and heating chamber 8; an upper side opening 11 in a sidewall of the cooking chamber 6 through which the upper end (typically an outfeed section) 13 of the conveyor 10 extends; a lower sidewall opening 15 through which the lower end (typically an infeed section) 17 of the spiral conveyor 10 extends; a circulation fan 14 for circulating a heating medium (e.g., air, combustion gas, steam, or a combination thereof) 5 through the spiral oven 2; an exterior fan motor 19; one or more indirect heat exchangers, fuel burners, electric elements, thermal oil plated or finned heating exchangers, or other heating devices 21 within the upper circulation and heating chamber 8 for heating the cooking medium 5 used for cooking the food product.
In operation, the food cooking medium 5 is delivered by the circulation fan 14 through the heating device 21 provided in the upper chamber 8 and then is discharged vertically downward into the cooking chamber 6 via either one or more openings in the cooking chamber ceiling 12 or one or more discharge cones 23 extending a short distance downwardly into the cooking chamber 6 above the elevation of the upper end 13 of the conveyor 10. Within the cooking chamber 6, the circulation fan 14 operates to draw the cooking medium 5 inwardly across the 360° spiral flights 18 for convective cooking and then into a vertical return flow path 20 which is surrounded by the spiral conveyor 10.
The spiral conveyor 10 can convey the food product 16 either upwardly or downwardly in the cooking chamber 6 during the cooking process but will typically convey the food product 16 upwardly (a) so that drippage from the raw or partially cooked product entering the oven is not allowed to fall on and contaminate the cooked product exiting the oven and (b) so that the cooked fat which drips from the cooked product will fall on and baste the product as it travels along the lower flights. The spiral conveyor 10 has an axis of rotation (i.e., an axis of spiral travel) 24, typically also corresponding to the longitudinal axis of the vertical return flow passageway 20, around which the food product 16 is conveyed as it is contacted by the cooking medium 5 during the cooking process.
In another type of prior art spiral oven, a plenum is provided outside of one or two side walls of the cooking chamber to draw the cooking medium out of the cooking chamber. Typically, the side wall of the cooking chamber will have openings therethrough and will constitute the inner wall of the plenum. The oven circulation fan(s) will also typically be located inside the plenum.
Heretofore, spiral ovens have had significant shortcomings which have significantly limited the types of cooking operations which can be performed in these ovens and have resulted in (a) significantly non-uniform flow, temperature, and cooking conditions within the cooking chamber 6 and (b) non-uniform cooking results which vary, for example, depending upon whether the food product is conveyed near the inside edge 28, near the outside edge 26, or on the central portion of the spiral conveyor belt 10.
As seen in FIG. 1, the circulation fan 14 of the prior art system 2 operates to draw the cooking medium return flow 27 upwardly through the center of the cooking chamber ceiling 12 and then operates to discharge the heated cooking medium 5 vertically downward into the uppermost end of the cooking chamber 6 through the one or more ceiling openings or cones 23. As will be apparent, the path of least resistance (i.e., the path of lowest pressure drop) is for the cooking medium to simply flow directly through the upper portion of the cooking chamber 6 from the ceiling opening(s) or cone(s) 23 to the circulation fan 14. Consequently, flow rates and temperatures in the lower regions of the cooking chamber 6 can be significantly lower than the flow rates and temperatures experienced in the upper regions of the cooking chamber 6.
In addition, as would be expected with a circulation system of this nature, dead zones are also commonly created at various locations within the cooking chamber 6 where the cooking medium 5 does not freely circulate. Further, the flow patterns which occur across the belt from the outside edge 26 to inside edge 28 can result in the food products placed toward the interior edge 28 of the belt receiving less beneficial conductive heat transfer than the products placed toward the outer edge 26 of the belt, especially in the lower regions of the cooking chamber 6 where lower cooking medium flow rates are experienced. This, in turn, can result in significant differences in cooking uniformity and product appearance.
The limited and non-uniform convective cooking flow circulation systems and patterns provided by the prior art spiral oven 2 also result in slower cooking times and reduced product throughput. Because the convective cooking medium flow rate through the lower regions of the cooking chamber 6 is significantly less than the convective flow rate in the upper regions of the cooking chamber 6, much less cooking occurs in the lower regions of the cooking chamber 6 than in the upper regions. Consequently, the conveyor rate must be slowed so that the product 16 is allowed to spend more time in the upper regions of the cooking chamber 6 than would otherwise be required if the same flow rates and temperatures were experienced in the lower regions of the oven. Further, the conveyor throughput rate must also be slowed to ensure that the product placed closest to the inner edge 28 of the spiral conveyor belt 10 is adequately cooked.
In addition, the available cooking applications for the prior art spiral oven 2 have also been further significantly restricted because the prior art spiral oven 2 is limited to the lateral convective flow of the cooking medium across the spiral flights 18 of the oven 2. Spiral ovens 2 have thus not been able, for example, to provide the surface browning and texture benefits of an impingement-type oven which can impinge the hot cooking medium directly on the surface of the product at a much greater velocity. Rather, heretofore, to obtain any beneficial impingement effects associated with the use of a spiral oven 2, it has been necessary to use one or more additional impingement chambers which impinge hot air or other cooking medium onto the product 16 before it enters the spiral oven 2 and/or as the product is being conveyed from one spiral oven 2 to the next spiral oven 2 in a multi-stage cooking process.